(1) Field of the Invention
The present invention relates in general to electromagnetic switches and more particularly to an electromagnetic device used to control the operate and release characteristics of a magnetically activated electronic switch.
(2) Description of the Prior Art
Magnetically activated electronic switches or "Hall effect" digital switches utilize the Hall effect for sensing a magnetic field. That is, a Hall cell included in the switch senses a magnetic field and provides electrical outputs corresponding to the presence or absence of a magnetic flux. Such a Hall effect switch is the Sprague ULS-3006T which other than the Hall cell, includes an amplifier, trigger and output stages integrated into a single monolithic chip. These relatively inexpensive solid state devices are compact and highly reliable. Further, since switching is done without the need of contacts they are not prone to contact bounce and thus are well suited in digital electronic applications.
Operationally these switches are configured in either a normally "off" or a normally "on" configuration. In the normally "off" configuration an output transistor in the output stage of the switch is normally off when the magnetic field perpendicular to the surface of the Hall cell is below a threshold or "operate point". When the field exceeds the "operate point" the output transistor switches on and is capable of sinking a set amount of current. The output transistor switches off when the magnetic field is reduced below the "release point" which is less then the "operate point". The normally "on" devices output transistor sinks current and is on when the magnetic field is below the threshold or "operate point" and switches off when the field exceeds the "operating point". The hysterisis characteristics provide for unambiguous or non-oscillatory switching regardless of the rate of change of the magnetic field.
These devices are finding increased usage as sensor devices which can detect either a current or electrical potential and trigger a signal such as to a digital computer or to an alarm device. Such an application is described in U.S. Pat. No. 4,104,488 titled "Subscriber Line Circuit" to D. A. Weir et al, issued May 19, 1977. The Weir patent teaches Hall effect devices which are incorporated to detect line looping, dialing and ring-trip sensing in a telephone system.
A problem associated with the Hall effect switches is that the sensitivities or the operate and release thresholds of the devices vary from device to device.
This can be solved by purchasing a large number of devices, testing and using only those devices which fit certain circuit characteristics. Or on the other hand designing the circuit characteristics around a specific operate and release sensitivity and fiting the appropriate Hall switch to these characteristics. In any event, these methods are costly and wasteful in both material and engineering which ultimately would make a final product very expensive.
One method of solving this problem and therefore the object of this invention is to provide an electromagnetic device that is capable of varying the amount of magnetic flux delivered to the Hall effect switch in order to tailor the operate sensitivities of the Hall effect switch to the desired operate and a non-operate characteristics of the electromagnetic device.